In industrial systems and in automated systems, it is of particular significance that the feed voltages of the individual loads or load groups are supplied according to special safety criteria. In particular, it is of primary importance that information processing parts of a control system, such as microprocessor subassemblies, must be supplied with the needed power in the event of a power failure. In many industrial applications, the feed voltage is 24 volts d.c., but other d.c. values are also typical, and even a.c. voltages, such as 115, 230 or 24 volts, are used.
Short circuits or overloads in partial areas of a control system, for example, in the output peripherals, can easily result in a complete, albeit usually only brief, failure of the feed voltage of the control system, which often leads to a loss of data in the central control units that are supplied by the feed voltage.
The increasing use of switched mode power supplies highlights this problem since, because of the sensitive electronics, the internal control circuits limit the output current to values only slightly higher than the nominal current. There exists in particular the problem that conventional fuses for the individual outputs are unable to disconnect in a sufficiently brief period of time. For rapid disconnection, fuses or circuit breakers often require a multiple of their nominal current. However, the switched mode power supplies cannot supply this in addition to the remaining load, so that the total feed voltage bursts in—even before a fuse triggers and the faulty output or branch is cleared.
According to the older application PCT/AT 00/00318 of the applicant, the use mentioned at the outset of controlled semiconductor switches is therefore provided. A circuit of this type is explained below in relation to FIG. 1.
According to FIG. 1, a switched mode voltage transformer SPW supplies an output or feed voltage Us, for example 24 volts, to ground. Such voltage transformers or switched mode power supplies are known to one skilled in the art in a multitude of designs and in and of themselves do not constitute the subject matter of the present invention. For the most part, an input a.c. voltage, for example 230 volts, is rectified and the resulting d.c. voltage is fed via a cycled switch to a primary winding of a transformer. A rectification of the feed voltage is repeated in secondary circuit. Voltage transformer SPW operates, for example, as flyback or flux converter and is usually regulated to a constant output voltage. However, it must be emphasized that the invention is not confined to specific transformers and that the feed voltage, for example can also be a regulated or unregulated a.c. voltage.
Feed voltage Us is fed to a first output A1 via a conventional fuse Si1 a controlled switch SW1 and via a measuring shunt RM1. Output voltage UA1 is applied to output A1. In the same manner, feed voltage Us is fed to an output A2 and an output A3. Fuses Si1-Si3 are, for example, fusible cutouts and are provided primarily if this calls for standard safety determinations, primarily for fire protection. However, they are not of any significance for the function of the invention.
Fed to a monitoring unit UWE are, on the one hand, feed voltage Us, which is compared in a comparator KOM to a reference voltage URef, and, on the other hand, any of the voltages occurring on the measuring shunts that are proportional to the output voltages, for example Ui3r, for the comparison in switching amplifiers, for example SV3. The shown circuit, when connected to a sequencing control (not shown), makes it possible to selectively disconnect outputs, when the feed voltage bursts in, according to predetermined priorities or to disconnect individual branches in the event of an overload. It is also possible to additionally or alternatively monitor the output voltages UA1 . . . UA3 and to call on them for disconnect operations. The control state in question may be indicated, for example, by little lights L1 . . . L3.
During normal operation, a switching transistor of a long branch is in an operating state in which only a slight drop in voltage occurs; that is, the transistor is in saturation when there is a longitudinal voltage of, for example, clearly under one volt. Accordingly, the power dissipation of the transistor is not all that great. The cooling unit is also designed for such a continuous power dissipation.
Then, if the limit current is exceeded, the transistor—together with the monitoring circuit—begins to hold the output flow constant in order not to overload the feed source too much and thus provoke a crash of the entire system. The saturation state is left and as a result a high power dissipation is then carried out in the transistor, specifically in the extreme case short circuit current times feed voltage. Because the transistor, for example of the MOSFET type, has already been heated up during normal operation, the absorption of the additional power loss in the limit state is problematic. On top of that, the limit state lasts a short period of time, for example 50-100 ms, then the disconnection occurs. In this short time, the heat cannot at all be passed on to a heat sink and therefore must be absorbed by the transistor chip. In order to prevent destruction of the switching transistor before disconnection, transistors having very large chip surfaces must therefore be used, which results in high costs.
A similar circuit emerges from German Patent 299 09 206 U1, which describes a protective device for a low-voltage current distribution system in which each circuit is assigned its own circuit breaker having adjustable current limitation as short-circuit and/or overload protection. The break time of the individual switches may be affected by the longitudinal voltage across these switches.
U.S. Pat. No. 5,969,514 describes a buck converter in which the otherwise typical single switching transistor is implemented using transistors that are connected in parallel with drain and source and are started or not started or triggered as a function of the level of output voltage. Cycled switching transistors are always present in this case; however, the set of problems involving a disconnect fuse, which in principle is only switched on or off, is not present.
An object of the present invention is thus to provide a power supply in which the problem of expensive overdimensioning of the semiconductor switch no longer exists.
Starting from a power supply of the type mentioned at the outset, this objective is achieved according to the invention in that at least one auxiliary semi-conductor switch is connected in parallel that, in the event of an overload, absorbs a substantial portion of the overload current in the branch.
By employing the invention, the thermal “surge” is thus quickly transferred, before disconnection, onto the auxiliary transistor, which for this purpose is available without the aforementioned preliminary increase in temperature.
In an expedient variant, provision is made for the monitoring unit to be set up to keep the auxiliary semiconductor switch at least essentially disconnected during normal operation, but to switch it on in the event of an overload as the main semiconductor switch is simultaneously disconnected. In this way it is possible to affect the characteristic of the transition from the main switch to the auxiliary switch by appropriate design of the monitoring unit.
It is especially advantageous if a ballast resistor is connected in series with the auxiliary semiconductor switch. In this way, the majority of the heat is dissipated in the ballast resistor and the auxiliary semiconductor switch can be dimensioned for a lower power dissipation and therefore be cheaper. In the process it is advisable if the given short circuit current of the branch is determined essentially by the ballast resistor and the feed voltage so that R1A≈Us/IK1.
In practice, it is expedient and economical if the semiconductor switches are of the FET type. In this manner, the triggering by the monitoring unit may be simplified. In so doing an especially simple solution in terms of circuitry is achieved if the semiconductor switches are of the self-locking FET type, the gate of the main semiconductor switch being connected to the source and triggered by an output of the monitoring unit via a Zener diode, and the gate of the auxiliary semiconductor switch being directly triggered by the same output. A ballast resistor, which is formed as a composite carbon resistor, is especially well-suited to absorb the power impulse that occurs when there is a disconnection.